415. Add Strings

Given two non-negative integersnum1andnum2represented as string, return the sum ofnum1andnum2.

Note:

1. The length of bothnum1andnum2is < 5100.
2. Bothnum1andnum2contains only digits0-9.
3. Bothnum1andnum2does not contain any leading zero.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

Code:

class Solution(object):
    def addStrings(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        ans, carry = [], 0 
        i, j = len(num1) - 1, len(num2) -1

        while i >= 0 or j >= 0 or carry == 1:
            a = 0 if i < 0 else ord(num1[i]) - ord('0')
            b = 0 if j < 0 else ord(num2[j]) - ord('0')
            s = a + b + carry
            ans.append(str(s % 10))
            carry = s/10
            i-=1; j-= 1

        return ''.join(ans)[::-1]